Its hard to say one thing or other since many of his games were groundbreaking and industry changing.If you were to list a game as his greatest achievement based sonely on its importance and historical context, I would say then Super Mario Bros as the best.Not only there was nothing like it back then, it went and brought back the industry from its knees.Not, if I were to say which one is the best based on the quality of the game, I would say Ocarina of Time, though Super Mario 64 comes close.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1